help !!! guitar amp feedback

[doozerface]

I play a jcm 800 50 watt single channel guitar head and an sg with a marshall 4x12.

I havent changed anything and out of nowhere I am experiencing intense feed back problems. Not the kind where you get close to the amp and it feedsback, but this horrible high pitched piercing sound. I've checked all cables, and the guitar is cool but i cant figure out the problem. I have not opened up the amp. This feedback noise also occurs while im playing and at low and high volumes.

I plugged in my friends epiphone les paul and then the problem doesnt occur, i also then plugged my sg into his fender vibroverb and there is no feedback. I'm lost !

[goldsoundz]

I plugged in my friends epiphone les paul and then the problem doesnt occur, i also then plugged my sg into his fender vibroverb and there is no feedback. I'm lost !

i really don't think you should be

[soundguy]

When a preamp tube decides to die (its imaginable that power tubes might do the same thing however I dont have experience there) they can exhibit a high pitched run away squealing that is sort of similar to the sound of a guitar feeding back when you stand near the speaker. Its typically all high end and distinctly tube sound, but its easy to confuse it I suppose if youve never been confronted with it before. Try to change your preamp tubes and see if it goes away. Put good tubes back in there...

dave

[vvv]

What soundguy said.

[doozerface]

preamps, i'll check that out.. thanx

[Knights Who Say Neve]

What you describe can be caused by a microphonic preamp tube, i.e. the tube innards are vibrating, causing feedback. You can just power up the amp and tap the preamps tubes with a chopstick or wood pencil. If one of them is microphonic it will make noise when you tap it.

[soundguy]

well, most tubes in any guitar amp that I have ever owned are gonna make noise if you tap them with the amp on...

dave

[doozerface]

do you think it could have anything to do with my guitar? i plugged another guitar in and the feedback was gone. I looked at the tubes last night, took them out and they look good. i'll try it out again when i get outta work.

[vvv]

If it is a pre-amp tube, it is possible that one guitar may have a greater problem than another.

As far as tapping the tubes, a microphonic tube will often (not always) make an obviously "bad" sound; it may squeal or "thunk, " etc., especially in comparison to other tubes. Sometimes, also, the color might be a clue.

Pre-amp tubes (typically 12AX7's and the like) are pretty cheap, and it's always a good idea to have a couple of spares around. Swapping them out is an easy test, and you might just like the change of tube.

While the problem you describe doesn't sound like a power tube to me, I'd check those, also.

Incidentally, an eraser on the end of a pencil, or a chop-stick are good "tappers".

[sloanfiske]

i'd guess its the pickups on your sg. the sg's i've had have always had a bit of bite and would feedback.

those pickups are wound to be pretty hot and that body is pretty thin.

if you plug in another guitar and nothing happens...that's my guess.

your amp is fine. don't tap the tubes...just turn down the amp.

although I could be wrong and the feedback could be microphonics...in which case...you need a new preamp tube or two. they'll look fine...but they're just fooling you.

[doozerface]

As far as my guitar goes, i checked all the connections and they are fine. I also lowered my pickups and nothing has changed. I opened up the back of my head and to my eyes all the tubes looked fine. I then plugged the amp back in and noticed that 2 of the 3 smaller tubes(im assumin the preamp) were not glowing as brightly as that other one. I then plugged my guitar back in to see if anythign changed but it hadnt. I started messin with the knobs on the amp and noticed that when i turned the treble down a good amount that there was no feedback. I think most feedback comes from treble frequencies but like i said this is not normal feedback noise.

[kayagum]

As far as my guitar goes, i checked all the connections and they are fine. I also lowered my pickups and nothing has changed. I opened up the back of my head and to my eyes all the tubes looked fine. I then plugged the amp back in and noticed that 2 of the 3 smaller tubes(im assumin the preamp) were not glowing as brightly as that other one. I then plugged my guitar back in to see if anythign changed but it hadnt. I started messin with the knobs on the amp and noticed that when i turned the treble down a good amount that there was no feedback. I think most feedback comes from treble frequencies but like i said this is not normal feedback noise.

Yeah, your EQ is a band-aid fix, but are you basically saying you're too cheap to replace a couple of 12ax7 tubes?

[doozerface]

at no point did i say that. Im just sayin i noticed that when i turned the treble down it made the feedback go away. I was hopin someone might know if that was more of a hint that it had something do with my amp more than guitar or if lowing treble always made amps feedback less. Im not all that knowledgeable on this whole subject. Im here to figure out what is wrong with the amp and fix it.

[kayagum]

Sorry for my smart-ass remark, but the majority of the answers you got (some from very respected/reputable posters like soundguy) related to the preamp tubes, and you chose to ignore it.

It's a common phenomenon on the board: people ask for advice, they get it, and they choose to ignore it and go with what they already do.

If you're serious about figuring this problem out, buy some 12ax7 tubes, swap them, and see what they do. At the very least, even if that doesn't help your problem, you'll have some spares, which is a good thing to have.

If lowering the treble works, it works. If your preamp tubes crap out, don't say we didn't warn you.

[stinkpot]

To reiterate what has already been posted. Open up your amp, turn it on with it plugged into the cabinet, and lightly TAP each preamp tube with the aforementioned instument(s). If the tubes are good, you shouldn't hear much of anything comming from the cabinet, if they're bad, you'll hear the TAPPING through the speakers. Just looking at them doesn't do any good.
Also, make sure that you remove the preamp tube covers. Usually you push down a little and twist.
TAP your tubes is the operative advice of this thread.
Good luck.